A signal in a transmitter undergoes a number of stages, such as modulation, up conversion and amplification, before actually being transmitted. A transmitter in a wireless circuit usually comprises a variety of circuit elements such as frequency mixer, local oscillator, power amplifiers, filters and antennas.
With reference to FIG. 1, a block circuit diagram of a conventional transmitter 10 is shown. The conventional transmitter 10 in FIG. 1 includes a baseband source 12, an up-converter (comprising a local oscillator 14 and a mixer 16), a power amplifier chain 18 and an antenna 20. In the transmitter circuit 10, the baseband source 12 produces an intermediate frequency (IF) signal. The IF signal is provided to the up-converter, which converts the IF signal to a radio frequency (RF) signal. The RF signal is provided to the power amplifier chain 18 which amplifies the signal and finally the signal is transmitted through the antenna 20. The power amplifier chain 18 generally includes preamplifiers and power amplifiers.
As wireless communications system evolves, the demand for a light weight wireless terminal with longer battery life also increases. Power amplifiers typically dominate the power consumption of these terminals. Hence, there is also an increased demand for low distortion and highly efficient RF power amplifiers to be implemented in the transmit chain of a wireless terminal.
In order to increase power efficiency, amplifiers need to be driven close to their saturation region, where they have a tendency to non-linearity and there may be significant levels of distortion. Distortion can be reduced by backing off the amplifier from saturation, but this reduces the power efficiency. There is an inherent trade-off between linearity and efficiency.
It is important that each stage of the amplifier chain have adequate linearity with minimum distortion. Therefore, power amplifiers are typically operated in class-A or class-AB configuration, which also implies low efficiency. Other classes of power amplifiers, such as class B power amplifiers, show high efficiency but are often not suitable for linear applications. Hence, it is common to employ the Kahn Envelope Elimination and Restoration (EER) technique with efficient power amplifiers to achieve linear amplification. The Kahn Envelope Elimination Technique is based on combining a highly efficient but nonlinear RF power amplifier with a highly efficient envelope amplifier to implement a high-efficiency linear RF power amplifier. A detailed description of the EER transmitter is given below.
FIG. 2 shows the architecture of a conventional EER transmitter 30. A modulated RF signal is provided at the input terminal of the EER transmitter 30. The RF signal is separated into two separate forward paths by a splitter 32. In the upper path as illustrated, an envelope detector 34 detects the input signal and generates envelope information, E(t). A sigma-delta/pulse-width modulator 36 then digitises the envelope information, E(t), providing an output to a class-S amplifier 38, which amplifies the digitised envelope. The digital output from the class-S amplifier 38 is then filtered by a low pass filter 42 which converts the digital envelope signal back to analogue. The analogue envelope signal is used as the supply voltage for a power amplifier 44. In the lower path, a limiter 40 detects the phase of the input signal. The limiter 40 produces a constant envelope output signal, which is then amplified by the aforementioned power amplifier which is selected to be a power efficient, but highly non-linear, switching power amplifier. The output power of the switching power amplifier 44 is proportional to the DC supply voltage, i.e. Pout∝V2DD. Since the supply voltage, VDD, of the power amplifier 44 is derived from the envelope signal, this restores the original envelope of the input signal at the power amplifier stage.
However, one of the major drawbacks of implementing an EER transmitter is that it can only be implemented for narrowband applications. This is generally due to the limited bandwidth and low frequency operation of the class-S amplifier stage. Another disadvantage of implementing an EER transmitter is that it cannot be implemented for high peak-to-mean ratio (PMR) communication standards such as wireless local area network (WLAN). This is because high PMR signals require the supply voltage of the power amplifier to be modulated over a large dynamic range which introduces non-linearity.
Another known method of achieving linear and efficient amplification is through the use of Class-S amplifier as a main power amplifier. As shown in FIG. 3, the sigma-delta/pulse-width modulator 52 digitises the input RF signal. The digitised output waveform is then used to drive the Class-S amplifier 54. Finally the original RF signal is restored at the output of a band-pass filter (BPF) 56. However, this method suffers a drawback, in that with the current technology it is difficult to digitise an RF signal.
A transmitter architecture was proposed in Yuanxun Wang, “An improved Kahn transmitter architecture based on delta-sigma modulation”, IEEE International Microwave Symposium Digest, Volume 2, June 2003, Page 1327 to 1330. The transmitter architecture described is based on combining the EER concept and the digital modulation concept mentioned in the previous paragraph. Referring to FIG. 4, the envelope and carrier of the input RF signal is split into two separate paths. The envelope signal is digitised using a delta-sigma modulator 66 and the digitised envelope is directly modulated on a carrier using an RF mixer 72. The final RF output is obtained by passing the modulated signal through a class-S power amplifier 74 and a band pass filter 76.
A similar transmitter architecture was also proposed in Alexandre Dupuy and Yuanxun Ethan Wang, “High efficiency power transmitter based on Envelope Delta-Sigma Modulation (EDSM)”, Vehicular Technology Conference, Volume 3, September 2004, Page 2092 to 2095. In this transmitter circuit, a Class-E power amplifier is employed instead of a Class-S power amplifier.